Not Applicable
The present invention is directed to an OFDM/DMT digital communications system. More particularly, the present invention is directed to an apparatus and method for synchronizing the clocks used in a transmitter and receiver of an OFDM/DMT digital communications system. The present invention is particularly applicable in multipoint OFDM/DMT digital communications systems.
Multi-point communications systems having a primary site that is coupled for communication with a plurality of secondary sites are known. One such communications system type is a cable telephony system. Cable telephony systems transmit and receive telephone call communications over the same cable transmission media as used to receive cable television signals and other cable services.
One cable telephony system currently deployed and in commercial use is the Cablespan 2300 system available from Tellabs, Inc. The Cablespan 2300 system uses a head end unit that includes a primary transmitter and primary receiver disposed at a primary site. The head end unit transmits and receives telephony data to and from a plurality of remote service units that are located at respective secondary sites. This communication scheme uses TDM QPSK modulation for the data communications and can accommodate approximately thirty phone calls within the 1.9 MHz bandwidth typically allocated for such communications.
As the number of cable telephony subscribers increases over time, the increased use will strain the limited bandwidth allocated to the cable telephony system. Generally stated, there are two potential solutions to this bandwidth allocation problem that may be used separately or in conjunction with one another. First, the bandwidth allocated to cable telephony communications may be increased. Second, the available bandwidth may be used more efficiently. It is often impractical to increase the bandwidth allocated to the cable telephony system given the competition between services for the total bandwidth available to the cable service provider. Therefore, it is preferable to use the allocated bandwidth in a more efficient manner. One way in which the assigned bandwidth may be used more efficiently is to use a modulation scheme that is capable of transmitting more information within a given bandwidth than the TDM QPSK modulation scheme presently employed.
The present inventors have recognized that OFDM/DMT modulation schemes may provide such an increase in transmitted information for a given bandwidth. Such systems, however, present a number of technical problems. One such problem is the determination of how one or more remote receivers are to synchronize their internal clocks and timing systems with the internal clock and timing system of a primary transmitter at a central site. A remote receiver must first synchronize its internal clock and timing system with the clock used by the primary transmitter to synthesize the transmitted signal before the remote receiver can properly demodulate the data that it receives. A further problem occurs in multipoint communication systems in which there are plural groups of remote transmitters that transmit data to centrallized transceivers. Each group of transmitters often has its transmissions frequency multiplexed with transmissions from other groups before being demultiplexed for receipt by a particular central transceiver. The resulting multiplexing/demultiplexing operations introduce frequency offsets for which compensation must be made if the receiver of the central transceiver is to properly extract the correct data from the signals that is receives. The present inventors have recognized the need for such upstream and downstream clock synchronization and have disclosed solutions to these problems.
A According to one apparatus embodiment of the invention, a communications system comprises a transmission medium, and a clock arranged to generate a clock signal. A transmitter is responsive to the clock signal and is arranged to generate a plurality of frequency bins, at least one of the frequency bins comprising a pilot sub-symbol, to combine the frequency bins into a symbol, and to transmit the symbol including the frequency bins. The system also comprises a receiver arranged to receive the transmitted symbol including the frequency bins, to recover the pilot sub-symbol from the frequency bins in response to a timing signal, and to adjust the timing signal in response to the pilot sub-symbol.
According to another apparatus embodiment of the invention, a communications system comprises a transmission medium, a first clock arranged to generate a first clock signal and a second clock arranged to generate a second clock signal at an adjustable frequency. A transmitter is responsive to the first clock signal and is arranged to generate a series of pilot sub-symbols by using at least a first value of a first parameter and to transmit the sub-symbols on the transmission medium. The system also comprises a receiver arranged to recover the pilot sub-symbols in response to the second clock signal, repetitively to calculate differences between the values of the first parameter among the recovered pilot sub-symbols, and to adjust the frequency of the second clock signal so that the differences decrease.
Another apparatus embodiment of the invention is useful for generating a pilot signal for synchronizing. In such an environment, the apparatus comprises a clock arranged to generate a clock signal and a transmitter responsive to the clock signal. The transmitter is arranged to generate a plurality of frequency bins, at least one of the frequency bins comprising a pilot sub-symbol, to combine the frequency bins into a symbol, and to transmit the symbol including the frequency bins.
Another apparatus embodiment of the invention is useful for synchronizing communication signals by using a series of pilot sub-symbols defining values of a first parameter. In such an environment, the apparatus comprises a clock arranged to generate a clock signal at an adjustable frequency and a receiver. The receiver is arranged to recover the pilot sub-symbols in response to the clock signal, repetitively to calculate differences between the values of the first parameter among the recovered pilot sub-symbols, and to adjust the frequency of the clock signal so that the differences decrease.
According to one method embodiment of the invention, communications are synchronized by steps comprising generating a clock signal at a first location, and generating a plurality of frequency bins at the first location in response to the clock signal. At least one of the frequency bins comprises a pilot sub-symbol. The frequency bins are combined into a symbol, and the symbol including the frequency bins is transmitted to a second location. The transmitted symbol including the frequency bins is received at the second location. The pilot sub-symbol is recovered from the frequency bins in response to a timing signal, and the timing signal is adjusted in response to the pilot sub-symbol.
According to another method embodiment of the invention, communications are synchronized by steps comprising generating a first clock signal at a first location, and generating a series of pilot sub-symbols at the first location in response to the first clock signal by using at least a first value of a first parameter. The sub-symbols are transmitted from the first location. A second clock signal is generated at an adjustable frequency at a second location. The pilot sub-symbols are recovered at the second location in response to the second clock signal. Differences between the values of the first parameter among the recovered pilot sub-symbols are repetitively calculated, and the frequency of the second clock signal is adjusted so that the differences decrease.
Another method embodiment of the invention is useful for generating a pilot signal for synchronizing comprising generating a clock signal and generating a plurality of frequency bins in response to the clock signal. At least one of the frequency bins comprises a pilot sub-symbol. The frequency bins are combined into a symbol, and the symbol including the frequency bins is transmitted.
Another method embodiment of the invention is useful for synchronizing communication signals by using a series of pilot sub-symbols defining values of a first parameter. In such an environment, the method comprises generating a clock signal at an adjustable frequency, and recovering the pilot sub-symbols in response to the clock signal. Differences between the values of the first parameter among the recovered pilot sub-symbols are repetitively calculated, and the frequency of the clock signal is adjusted so that the differences decrease.
Other features and advantages of the present invention will become apparent upon review of the following detailed description and accompanying drawings.